1. Field of the Invention
The present invention relates, generally, to methods and apparatus for controlling the dynamic flight characteristics of a ramjet missile, more especially to velocity controllers therefor, and most particularly to a velocity controller for a solid fuel ramjet missile which regulates velocity and maintains same substantially uniform. In one aspect of the present invention, it further relates to methods and means for reducing the drag experienced by a ducted rocket missile under subcritical inlet operation.
2. Description of the Background Art
Ramjet engines are, of course, well known. These devices are employed for powering aircraft, and particularly missiles travelling at supersonic speeds. Within this context, three basic types of ramjet engines are in use; viz., liquid fuel, ducted rocket, and solid fuel ramjets. Each partakes of the same general similarity of burning a fuel in a combustion chamber in the presence of air admitted thereto through or via a supersonic inlet. A principal distinction regards the manner in which the fuel is supplied to the combustor.
A liquid fuel ramjet employs a spray of fuel into the combustor wherein it is mixed and reacts with the oxidant air admitted through the jet inlet. Thrust is simply and easily regulated by control on the metering valves supplying fuel to the combustor. Ducted rockets rely upon the initial combustion of a fuel-rich propellant in a hot gas generator, whereupon the gases are thence injected into an aft combustor for further oxidation reaction with admitted air. The fuel flow, in the hot gas state, is essentially constant and independent of flight conditions. However, different situations obtain in respect of the combustion characteristics responsible for thrust (and hence velocity) of a solid fuel ramjet engine.
The propellant in a solid fuel ramjet, termed a fuel grain, is in the form of a hollow, shaped-sustaining mass of, e.g., a hydrocarbon, having a central port through which air may pass. Admission of air through a supersonic inlet to the central port provides the necessary oxidizer for combustion of the grain. The rate of combustion is dictated by the mass flow rate of oxidizing air through the grain and, accordingly, is sensitive, inter alia, to the speed of the aircraft, its altitude and the inlet size. To a certain extent, the fuel grain can be designed with limited pre-programming of flight parameters in mind by providing radial gradients in the physical and/or chemical characteristics of the grain to be responsive to anticipated flight conditions; but that is not always possible or even desirable.
Usually, the fuel in a solid fuel ramjet engine is combusted in two stages. The initial interaction between the fuel grain and the oxidizing air is typically incomplete, leaving a fuel-enriched gas. This gas is subjected to further oxidation in an aft section or stage prior to exiting the engine and providing thrust for the aircraft. This has led to one proposal for controlling the flight characteristics of, e.g., a missile employing a solid fuel ramjet engine, known as a variable bypass. That approach incorporates a dual flow path for inlet air, segregating it into a first portion routed to the combustor for contact with the fuel grain and a second portion routed to the aft section out of contact with the fuel grain for secondary combustion of incompletely burned fuel gases. While this approach has achieved some measure of control on the burning rate of fuel, the concept still leaves something to be desired. For example, the thrust developed by the solid fuel ramjet engine is proportional to the flow rate of gas through the exit port. The bypass approach admits the same volume flow rate of air through the inlet, merely apportioning the same between two ultimately recombining paths. Accordingly, thrust modulation is not completely achieved since total airflow remains constant.
U.S. Pat. No. 3,844,118 is generally representative of a type of bypass used to control the dynamic flight characteristics of a solid fuel ramjet by varying airflow to the solid fuel grain. That patent discloses a preference for the air inlet proximate the juncture of the fuel grain and the secondary or aft combustion section to avoid the need to install lengthy piping systems between the inlet and the secondary chamber to which the majority of inlet air is admitted. A forwardly-disposed return member routes primary combustion air along an isolated portion of the circumference of the fuel grain, through a flow controlling valve to the central port thereof. Adjustment of the valve apportions the primary and secondary airflows between that for combustion of the fuel grain and secondary combustion of fuel-rich gases, respectively. The U.S. Pat. No. 3,844,118 approach strives to provide thrust control simply on the basis of this dichotomy of flow and is implemented with subsonic airflow. Further, the location of the control valve, at the head of the port through the fuel grain, may adversely affect the flame out limits of the engine.
U.S. Pat. No. 3,220,181 is conceptually similar to the '118 reference, insofar as total airflow is constant and control is performed internally with subsonic flow. Although the mechanical implementation for apportionment differs somewhat, the same basic concept of bypass is relied upon.
U.S. Pat. No. 4,050,243 and British Pat. No. 669,014 both involve methods and means for controlling the flight characteristics of a solid fuel ramjet, and in a fairly similar way. The U.S. Pat. No. 4,050,243 patent allows subsonic airflow to reach the fuel grain in a type of on-off manner through a translatable tube-in-hole injector. The British Pat. No. 669,014 similar, excepting the fact that the on-off function is performed with supersonic flow at the front of the engine.
U.S. Pat. No. 4,307,743, assigned to the assignee of the present invention, is remarkable as respects the present invention. The U.S. Pat. No. 4,307,743 patent discloses a dynamic start device for an overcontracted, mixed compression, supersonic inlet for ramjet powered missiles. It comprises an air inlet opening for admitting external air into a duct leading to the ramjet engine combustor, wherein the inlet has opposed lip portions with one extending beyond the other, a cover hingedly fixed to the forward edge of the forwardmost lip and a support for holding the cover in a position where it extends between the lips to close the air inlet. A means is provided for releasing the support while the missile is at a supersonic speed, whereby the cover will be forced to rotate about the hinged connection and open the inlet by the force of air impinging on its outer surface. In that manner, dynamic start is achieved. Thus, that patent discloses an inlet with an interposed cover moveable from a first closed to a second open position; albeit, velocity control, per se, is not an objective of that patented device.
U.S. Pat. Nos. 3,439,692, 2,997,843, and 2,995,892, disclose various designs for air inlets used to admit combustion air to a jet engine. Of these, the U.S. Pat. No. 3,459,692 patent is perhaps most noteworthy insofar as it discloses an inlet structure permitting "air spilling" in order to match the available airflow to that required by a turbojet engine. The U.S. Pat. No. 2,997,843 patent is conceptually the same, save the fact that the turbojet airflow matching is automatically performed by an integrated pneumatic system. The U.S. Pat. No. 2,995,892 device varies the compression of airflow and, accordingly, influences the efficiency of the propulsion system. None of these devices, however, is used to vary in a direct manner the engine thrust as is the goal herein.
Returning to a brief consideration of ducted rocket missiles, as respects a related goal of the present invention, its sometimes occurs that back pressure in the combustor becomes so great as to force flow spillage in front of the inlet. This situation is termed "subcritical inlet operation"; wherein a subcritical inlet spills its flow by means of a normal shock system creating, in turn, a large incremental increase in missile drag. To date, a simple yet efficient means to overcome the unwanted and undesirable normal shock system accompanying subcritical inlet operation has yet to be devised.
Accordingly, and in light of the foregoing discussion, the need exists to provide a simple yet efficient and reliable means for controlling the flight characteristics of ramjet-powered missiles; in the context of a solid fuel ramjet, control over the velocity thereof and in the context of a ducted rocket, over subcritical inlet operation.